The large and primarily tropical plant family Acanthaceae contains at least 4000 species, placing it among the top 12 or so most diverse families of flowering plants. The group is part of the order Lamiales, which includes familiar plants such as snapdragons (Antirrhinum, Plantaginaceae), mints (Lamiaceae), African violets (Gesneriaceae), and monkeyflowers (Mimulus, Phyrmaceae). These plants have sympetalous corollas (i.e., the petals are fused into a tube) and four stamens (sometimes reduced further to two).

Acanthaceae includes the 221 genera treated by Scotland & Vollesen (2000) plus, in the broad sense (i.e., Acanthaceae s.l.), the black mangrove genus Avicennia. Notably, as a result of the largely tropical distribution of Acanthaceae, species-level diversity (as well as some generic diversity) remains poorly understood, and there is little doubt that many new species remain to be discovered. In particular, the Neotropics house the richest and most poorly documented angiosperm flora on Earth. It is thus not surprising that Neotropical Acanths remain incompletely known, and we expect many more species to be described from this region. As a quantitative indication of the pace of new discovery in Acanthaceae, Index Kewensis reports 81 new species described in the genus Justicia alone between 1986 and 1995. We expect the species count in the family eventually to exceed 4000.

Characteristics

Plants belonging to the core Acanthaceae or Acanthaceae s.s. (i.e., Acantheae through Ruellieae as indicated in thephylogeny); Acanthoideae sensu Scotland & Vollesen 2001) can be recognized by their fruits: few-seeded, explosively dehiscent capsule within which seeds are borne on hook-like structures called retinacula (the lignified derivatives of the funiculus). These retinacula are a unique and unreversed synapomorphy for Acanthaceae s.s.: among these plants–all 4000 of them–fruits vary in size and in seed number but are otherwise remarkably homogeneous. The hooks presumably serve to propel the seeds away from the parent plant when the fruit dehisces but comprehensive studies are lacking. See video footage of explosive fruit dehiscence in Acanthshere(filmed in collaboration with Dr. Dwight Whitaker at Pomona College).

Within core Acanthaceae, Acantheae are well marked as monophyletic by morphological and molecular synapomorphies. Its sister group, which we have here called Ruellioideae, is also marked by a number of morphological synapomorphies including cystoliths (crystals) in most plant structures. The Ruellioideae comprises probably ¾ of the diversity in the family and includes large genera such as Justicia, Ruellia, Barleria, Strobilanthes, and Dicliptera.

Outside of this core group, four small lineages are very closely related and are treated by most authors as belonging to Acanthaceae in the broad sense: the pantropical and subtropical black mangroves, Avicennia; Old World Thunbergia (including the commonly cultivated black-eyed Susan, Thunbergia alata) and related genera, Old and New World Mendoncia; and Nelsonioideae (Nelsonia, Elytraria, Staurogyne, a handful of other small genera). Each of these four groups has its own distinctive fruit type. Unfortunately, we have yet to identify a synapomorphy for these plus core Acanthaceae that is as notable and easy to recognize as fruit type is for the core group. Compared to most other Lamiales, Acanthaceae have relatively few ovules.

Morphological Diversity

Beyond fruits of Acanthaceae, diversity in other traits is spectacular. Plants are rosette and caulescent herbs, shrubs and trees (rarely vines except Thunbergia and Mendoncia which are almost always climbing plants). Most Acanths have solid green leaves, but a few have remarkably colored leaves, and some are cultivated for their foliage (e.g., polka dot plant, Hypoestes phyllostachya; Persian shield, Strobilanthes dyerianus). Inflorescences of many Acanths have large, colorful bracts that often eclipse the corollas in showiness (e.g., Pachystachys, Megaskepasma). However, bracts of plants of other species are diminutive, green and clearly not involved in attracting pollinators. Many (but not all) species have extremely showy flowers and a number are cultivated for their attractiveness (e.g., Justicia brandegeana and many other species belonging to this large genus, Aphelandra squarrosa, and Ruellia simplex). Across the family, flowers range in size from a few mm to more than 10 cm, with a parallel range of variation in shape and color. Variation in these corolla traits is no doubt associated with a wide range of animal pollinators (including bees, butterflies, flies, hawkmoths, hummingbirds, sunbirds, and bats), although studies are remarkably few.

Pollen and Chromosomes

Structural diversity continues below the visible level in Acanthaceae. In terms of number and type of apertures and, especially in terms of ultrasculpturing, pollen grains of Acanthaceae are without doubt as diverse as any other family-level lineage regardless of size. One species of Crossandra is proclaimed to have the largest pollen of any angiosperm, with grains reaching ½ mm in length.

Phylogenetic Relationships

There is at present little disagreement among students of Acanthaceae regarding membership in the family and delineation of the main clades as presented in thephylogeny. There is, however, quite a bit of work to be done to resolve relationships at a number of levels. For example, previous results have placed Avicennia sister to Thunbergioideae but without strong support from molecular data for this relationship and with no identifiable morphological synapomorphies supporting the relationship. Similarly, the placement of Neuracanthus as sister to Barlerieae + Whitfieldeae + Andrographideae is not strongly supported. Phylogenetic relationships among ca. 50 genera in the large lineage Ruellieae has yet to receive any broad-scale study. There also remains a great deal of work to be done on relationships within lineages.